1. Field of the Invention
The inventions disclosed and taught herein relate generally to concrete pavements; and more specifically relate to the efficient use of Portland cement concrete (“PCC”) in pavement design and construction.
2. Description of the Related Art
Soil is the unconsolidated, in-situ (in place) material upon which all pavements are constructed. The engineering, chemical and mineralogical properties of a particular soil can vary based on its geological history, such as its parent material (rock type such as limestone, sea shells, granite, etc.), how it was deposited (glacial, water-lain, wind blown, residual), grain size distribution (boulders to microscopic), etc. The engineering properties of a soil affecting pavement performance include strength, swell potential, soil permeability, moisture content, erodibility, and mineralogy. These properties and others will vary even within the same soil type or formation. Commonly, a pavement project, such as a roadway, will cross over multiple soil formations and, as such, the properties of the soils can vary significantly over the breadth of the project. The performance of a pavement may depend on the soil properties on which it sits and how the designer takes the specific soil properties into account. For example, to reduce the soil swell potential, stabilizers such as cement, fly ash, and lime may be mixed with the soil. The effect of the soil stabilization can be dependant on the soil mineralogy, often times limiting the choice of stabilizer, or requiring more of it. Additionally, soluble sulfates may exist in portions of the soil and when mixed with a calcium based stabilizer, may experience significant heave, which may cause damage to the pavement. As well, organic particles within the soil may require increased levels of stabilizers. All of the above, along with other factors, may be considered when designing and building pavements.
Historically, on the one hand, asphalt, or flexible, pavements tend to have a lower initial installed cost as compared to concrete, or rigid, pavements. On the other hand, concrete pavements tend to have a longer life cycle, lower maintenance costs and lower costs of ownership over periods of time. The conceptual design for asphalt pavements typically involves a life expectancy of approximately 7 years before scheduled maintenance. Scheduled maintenance may include milling the asphalt surface and placing a 2″ overlay of asphalt thereon. This maintenance is designed to last another 7 years before repeating the mill and overlay steps. This concept has become known as “staged construction.” Recently, the asphalt industry introduced a higher performance material referred to as “perpetual asphalt.” Perpetual asphalt typically costs about the same as a concrete pavement. While perpetual asphalt is touted to outlast and outperform densely graded asphalt, it may not last as long or enjoy the low maintenance costs associated with concrete pavements.
Concrete pavements have been designed to perform with little or no maintenance for 30 or more years. There are three basic types of concrete roadways. Jointed Plain Concrete Pavement (JPCP) may have transverse joints spaced less than about 17 feet (5 m) apart and may have no reinforcing steel in the roadway. JPCP construction may, however, contain steel dowel bars across transverse joints and steel tie bars across longitudinal joints.
Jointed Reinforced Concrete Pavement (JRCP) has transverse joints spaced about 30 to 40 feet (9 to 12 m) apart and contains steel reinforcement in the roadway. The steel reinforcement is designed to hold together any transverse cracks that develop. Dowel bars and tie bars are also used at transverse and longitudinal joints.
Continuously Reinforced Concrete Pavement (CRCP) has no regularly spaced transverse joints and contains more steel reinforcement than JRCP construction. The high steel content affects the development of transverse cracks and holds these transverse cracks together.
It is estimated that about at least 70 percent of the states in the United States build JPCP roadways, about 20 percent of the states build JRCP roadways, and about six or seven state highway agencies build CRCP roadways. Texas, for example, requires CRCP on streets or roadways with speed limits greater than 45 mph. CRCP roadways typically cost about 20% more than JPCP roadways.
A number of standard design guidelines for pavements have been and are being developed for pavement design and analysis. For example, the most widely used pavement design guidelines for the design of concrete roadways is the Guide for Design of Pavement Structures published in, for example, 1986 and 1993 by the American Association of State Highway and Transportation Officials (AASHTO '86; AASHTO '93). Another procedure for the design of concrete roadways includes the use of the Mechanistic-Empirical Pavement Design Guide and software (MEPDG), sponsored by the AASHTO Joint Task Force on Pavements, and which is currently being developed and tested by a number of individuals and entities throughout the United States for use in design and forensic evaluation of pavements. The contents of each of these pavement design guidelines are incorporated herein by reference for all purposes. The AASHTO procedures require a prediction of the number of 18,000 lbf Equivalent Single Axle Load (“ESAL”) that the pavement will experience over its design life. It is typical to use an ESAL of 20 million or more for a Portland cement concrete roadway. Other pavements design guidelines may also be employed as required by a particular application. For example, a set of guidelines published by the American Concrete Institute may be used for the design of a parking lot, driveway, or elevated concrete structure.
The design thickness of a concrete pavement, such as a roadway or parking lot, may be selected to allow long-term performance under a forecasted traffic volume with a given soil (substrate) condition. For example, when CRCP pavement is specified in Texas, the accepted road design historically requires a 12-13 inch uniform thickness of CRCP from the beginning to the end of the proposed road (lane mile) and across the roadway width. In contrast, Texas does not require this thickness (volume and uniformity) for asphalt pavements.
Furthermore, it is well known that substrate plasticity issues and/or sulfate issues can promote heaving, which may negatively impact the integrity of the pavement life cycle. In addition, variations in the water table can negatively affect concrete pavement design and performance. Requiring a uniform pavement thickness (e.g., 12″-13″) and continuous rebar placement throughout the pavement is a low-tech method of addressing varying conditions of the substrate.
Indeed, under the AASHTO design procedure, the modulus of the subgrade/subbase reaction (i.e., K-value) has a minimum effect on the designed roadway thickness. That is, a worst-case thickness design, such as may be required in some states, does not match the actual substrate conditions to the roadway design. In lieu of this, most designs establish a uniform thickness to compensate for variability in the substrate. This means that most concrete pavement designs are over-engineered and, therefore, overly expensive since the design is for the worst section of pavement and does not take into account the varying substrate conditions. This worst case design methodology typically carries a high initial installation cost (especially when compared to densely graded asphalt as an alternate design). With limited budgets, agencies, such as state departments of transportation, contractors, and others tend to choose the lower initial cost of an asphalt design, without respect to higher maintenance or life cycle costs.
The inventions disclosed and taught herein are directed to an improved concrete pavement system and method of designing an improved concrete pavement system.